The present invention relates to semiconductor packaging, and in particular, to an organic substrate for flip chip semiconductor packages.
Referring to FIGS. 1 and 2, a flip chip bonding configuration for a semiconductor package 10 that includes a semiconductor die 12 that is mechanically and electrically connected to a supporting substrate 14. Metallic bumps 16 on the bottom surface 18 of the die 12 overlap with metallic pads 20 correspondingly positioned in a die bonding region 22 located on the top surface 24 of the substrate 14. Typically, the metallic bumps 16 are connected to the underlying pads 20 by solder 26, thus providing direct electrical coupling between the die 12 and substrate 14. The direct electrical interconnection between the die 12 and substrate 14 provided by flip chip bonding configuration 10 advantageously eliminates the need for bond wires and results in low resistance and inductance values. Thus, flip chip bonding configurations provide for better signal integrity and power distribution to and from the die 12 than in other bonding technologies such as wire bonding or tape automated bonding.
The bottom surface 18 of the die 12 may include hundreds of metallic bumps 16 with each metallic bump providing an electrical interface to and from the die 12 for various electrical signals, core power, input/output (I/O) power, and ground. Electrical signals, core power, and ground are provided to the die""s internal logic circuitry (not shown). Similarly, the I/O power and ground are provided to the die""s I/O interface circuitry (not shown), i.e., input and output drivers. The metallic bumps 16, and therefore the corresponding pads 20 on the top surface 24 of the substrate 14, generally are arranged in concentric rectangular rings (not shown) and are spaced apart to prevent electrical shorting between adjacent metallic bumps/pads. However, the metallic bumps 16 and pads 20 may be configured in various patterns other than concentric rectangular rings.
The substrate 14 may be a multi-layer printed circuit board, ceramic substrate or another semiconductor chip. Typically, pads 28 on the bottom surface 30 of the substrate 14 are bonded, e.g., via solder 32, to another printed circuit board referred to as a motherboard 34. The substrate 14 provides for a mechanical and electrical interface between the often densely-packed metallic bumps 16 on the bottom surface 18 of the die 12 to less-densely packed pads (not shown) on the motherboard 34. The pads 28 on the substrate""s bottom surface 30 are placed so as to correspond to the pads (not shown) on the motherboard 34.
The various layers of the substrate 14 are formed by the well known processes used to create integrated circuits and printed circuit boards. The individual layers may be comprised of conductive or semiconductor material. Often, the conductive material is a metal, i.e., a copper-based material, which is plated onto semiconductor material layers and patterned by photolithographically removing deposited metal to form pads and traces. The substrate 14 may be referred to as organic if organic material is combined with the copper-based material so as to provide thermal expansion characteristics close to those of the motherboard 34 and to improve reliability in board assembly.
Traces (not shown) electrically interconnect pads 20 or 28 on either the top surface 24 or bottom surface 30 of the substrate 14 or to vias (not shown) which are holes that facilitate the electrical interconnection of the various layers of the substrate 14. The vias may be formed, for example, by mechanical or laser drilling a via hole through a layer or layers of the substrate 14. Surrounding the via hole on each level may be a circular land (not shown) made of a conductive material. Typically, the via hole is filled with a conductive material so that via lands surrounding the via holes on various layers of the substrate are electrically interconnected. A via may extend through to the bottom surface 30 of the substrate 14 and connect to a pad 28. Thus, the pads 20 and 28 on the top surface 24 and bottom surface 30, respectively, of the substrate 14 are interconnected to one another by means of traces and vias. Solder balls 32 are individually connected to each pad 28 for soldering the pads 28 on the bottom surface 30 of the substrate 14 to the corresponding pads (not shown) on the motherboard 34. The solder balls 32 connected to the pads 28 on the bottom surface 30 of the substrate 14 are collectively called a ball grid array 36.
Various electrical characteristics are considered when designing the substrate 14 including maintaining a low DC resistance and AC inductance for power and ground connections to and from the die 12, and maintaining wide spacing between traces (not shown), pads 20 and 28, and via lands (not shown) to minimize the possibility of crosstalk or electrical shorting. Thus, with respect to power distribution for the die 12, the distance between the pads 20 on the top surface 24 of the substrate 14 for core power distribution and the logic circuitry which actually consume the power should be as short as possible.
Recently, many semiconductor packages have been designed using C4 flip chip bonding technology. An exemplary C4 bonding technology pad arrangement 38 is shown in the top plan view of FIG. 3. Referring additionally to FIGS. 1 and 2, the C4 bonding technology utilizes pads 40 arranged in an area array in the die bonding region 22 on the top surface 24 of the substrate 14 that are separated from one another by a pitch xe2x80x9cpxe2x80x9d. In FIG. 3, the pads 40 are positioned at lattice points 41 (only numbered along one row and one column), where each lattice point 41 is located below the midpoint of each pad 40.
In FIG. 3, the top two rows 42 and 44 of pads 40 from adjacent the die edge are rows of signal pads 46 that accommodate the transfer of various electrical signals to and from the die 12. The top two rows 42 and 44 of pads 40 closest to the die edge 48 include signal pads 46 because the majority of the logic circuitry within the die 12 is located proximate to the die edge 48 and signal traces (not shown) on the top surface 24 of the substrate 14 are typically routed outside of the die bonding region 22. The third and fourth rows 50 and 52, respectively, of pads 40 down from the die edge 48 include core power pads 54, for the transfer of power to the logic circuitry in the die 12, I/O power pads 56, for the transfer of power to the I/O interfaces in the die 12, and ground pads 58, for providing ground connections to the die. The fifth and sixth rows 60 and 62, respectively, of pads 40 down from the die edge 48 are again rows of signal pads 46. The seventh and eighth rows 64 and 66, respectively, of pads 40 are core power pads 54 and ground pads 58. Thus, power and ground pads 54 and 58, respectively, are intermingled with the signal pads 46 so as to avoid problems that occur when large numbers of signal pads 46 result in the power and ground pads 54 and 58, respectively, being located further than three to four rows away from the die edge. The three dots 68 in the upper left hand corner, upper right hand corner, and the bottom of FIG. 3 indicate that the number and spacing between the pads 40 included in the pad arrangement 38 can vary along the top surface 24 of the substrate 14 in those directions.
FIG. 3 indicates examples of the maximum distances between the die edge 48, where the logic circuitry is assumed to be located, and a core power pad 54 and an I/O power pad 56. A maximum distance between the die edge and a core power pad is approximately 3.354 p (3.354 times p), where p is the pitch or distance between the midpoints of adjacent pads 40. Also, a maximum distance between the die edge and an I/O power pad is approximately 4.272 p (4.272 times p). If p is 0.25 millimeter, the distance between the die edge and the core power pad reaches approximately 0.8385 millimeter, and the distance between the die edge and the I/O power pad reaches approximately 1.068 millimeters. These distances may result in increased DC resistance and AC inductance values.
FIG. 4 is a top plan view of underlying layers of vias 70 that connect to the pads 40 in FIG. 3. Again, the vias 70 are used to rout electrical signals, core power, I/O power, and ground potential to the layers of the substrate 14 below the top surface 24. Each via 70 includes a via hole 72 indicated by the inside circle 73 with an accompanying ring-shaped via land 74 which surrounds the via hole 72 as indicated by the second concentric circle 75. The vias 70 in each underlying layer are located directly below the corresponding pads 40 at the lattice points 41 on the top surface 24 of the substrate 14, and thus, utilize a pad-on-via structure. Accordingly, the top two rows 76 and 78 of vias 70 adjacent the die edge 48 are rows of signal vias 80 that accommodate the transfer of various electrical signals. The third and fourth rows 82 and 84, respectively, of vias 70 down from the die edge 48 include core power vias 86, I/O power vias 88, and ground vias 90. The fifth and sixth rows 92 and 94, respectively, of vias 70 down from the die edge 48 are again rows of signal vias 80. The seventh and eighth rows 96 and 98, respectively, of vias 70 are core power vias 86 and ground vias 90. The three dots 68 in the upper left hand corner, upper right hand corner, and the bottom of FIG. 4 indicate that the number and spacing between the vias 70 included in the arrangement of vias 70 can vary along each layer in those directions.
FIG. 5 is a top plan view of another C4 bonding configuration pad arrangement 100, based upon the pad arrangement 38 of FIG. 3, in which additional core power pads 54 and additional ground pads 58 are alternatingly placed in columns of core power pads 102 and columns of ground pads 104 at the interstitial points 106 (only numbered along one row and one column), where each interstitial point 106 is the mid point between four adjacent lattice points 41. Similar to FIG. 3, FIG. 5 includes signal pads 46, core power pads 54, I/O power pads 56, and ground pads 58. The three dots 68 in the upper left hand corner, upper right hand corner, and the bottom of FIG. 5 indicate that the number and spacing between the pads 40 included in the pad arrangement 100 can vary along the top surface 24 of the substrate 14 in those directions. As shown in FIG. 5, a maximum distance between a core power pad 54 and the die""s logic circuitry, presumably located proximate to the die edge 48, is only approximately 1.803 p (1.803 times p). Therefore, if p equals 0.25 millimeter, the distance between the core power pad and the logic circuitry will be approximately 0.4508 millimeter. Thus, by adding core power pads 54 and ground pads 58 at the interstitial points 106, the pad arrangement 100 of FIG. 5 offers improved power distribution performance over the pad arrangement 38 of FIG. 3.
Similar to the C4 bonding configuration of FIGS. 3 and 4, the C4 bonding configuration of FIG. 5 utilizes a pad-on-via structure as shown in the top plan view of underlying layers provided by FIG. 6. Again, signal vias 80, core power vias 86, I/O power vias 88, and ground vias 90, respectively, are used to rout electrical signals, core power, I/O power, and ground potential to the layers of the substrate 14 below the top surface 24. The three dots 68 in the upper left hand corner, upper right hand corner, and the bottom of FIG. 6 indicate that the number a spacing between the vias included in the via arrangement can vary along each layer in those directions. One problem associated with the C4 bonding configuration shown in FIG. 6, is that the minimum pitch between adjacent vias is only approximately 0.707 p (0.707 times p), or approximately 0.1767 millimeters when p equals 0.25 millimeter. This is a smaller pitch between vias than that of the C4 bonding configuration of FIG. 4. The reduced via pitch is of concern when routing traces (not shown) between vias as discussed below.
Process design rules are used during semiconductor packaging design to limit the placement of adjacent metallic regions relative to one another, and thus, ensure that short circuits and/or open circuits do not occur in semiconductor devices. Process design rule values are determined, at least in part, based upon inherent limitations in the precision of the photolithography and/or etching techniques used when forming the metallic layers of the substrate 14. FIG. 7 is a magnified top plan view of two signal traces 108 routed between two adjacent vias 70 for the C4 bonding configuration of FIGS. 3 and 4. As shown in FIG. 7, if the diameter of the via land 74 is approximately 0.11 millimeter, and the process design rule is 28 micrometers, the two traces 108 having a 28 micrometer trace width can be routed between the vias 70 leaving a 28 micrometer space between adjacent metal.
FIG. 8 is a magnified top plan view of a signal trace 110 routed between two adjacent vias 70 for the C4 bonding technology configuration of FIG. 5 and 6. In contrast to FIG. 7, only one trace 110 can be routed between the vias 70. Also, a finer design spacing value of approximately 22 micrometers, instead of 28 micrometers, is required in FIG. 8. Therefore, if the C4 bonding configuration of FIGS. 5, 6, and 8 is to be used, more layers and a finer design spacing rule is required, both of which increase the overall cost for the substrate 14. Thus, there is a need to reduce the distance between the core power pads 54 and the die""s logic circuitry without increasing the overall cost of the substrate by requiring more layers and finer design spacing rules.
In one aspect of the present invention, a substrate has lattice points and interstitial points. The substrate includes a surface, a plurality of pads located on the surface at interstitial points, and a plurality of vias located in the substrate only at lattice points.
In another aspect of the present invention, a semiconductor package includes a semiconductor die that has a die edge and a substrate connected to the semiconductor die that has lattice points and interstitial points. The substrate includes a surface, a plurality of pads located on the surface at interstitial points, and a plurality of vias located in the substrate only at lattice points.
In another aspect of the present invention, a method is disclosed for configuring vias in a substrate that has lattice points and interstitial points and for configuring pads on the surface of the substrate. The method includes locating pads on the surface of the substrate at interstitial points, and locating vias in the substrate only at lattice points.
It is understood that other aspects of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein is shown and described only exemplary embodiments of the invention, simply by way of illustration of the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.